1) Field of the Invention
The invention relates to method for recycling water in a melamine production process.
2) Description of the Prior Art
The starting material used for melamine preparation is virtually exclusively urea, which is converted to melamine either in a catalytic low-pressure process or in a high-pressure process without catalyst.
The high pressure process is mainly conducted in a melamine plant comprising a high-pressure part, a wet process for melamine purification and waste water treatment.
Melamine purification is usually necessary since by-products, degradation products and unconverted starting materials are present in the crude melamine. Since the melamine workup is typically affected in the presence of water, these by-products are obtained as wastewater constituents, for example as constituents of the mother liquor from the melamine crystallization.
The wet process usually comprises the steps of quenching the melamine melt with an aqueous phase and subsequent melamine crystallization. In order to obtain melamine of high quality an alkali containing quenching solution is used. The alkaline mother liquor can be subsequently either recycled or undergoes a treatment for removing melamine, by-products and degradation products.
Typically, the wastewater of a melamine plant comprises triazines, for instance the oxoaminotriazines ammeline and ammelide, melamine, melam, cyanomelamine, ureido-melamine, cyanuric acid, and also ammonia, carbon dioxide, urea and possibly NaOH in different amounts.
The prior art discloses some processes for treating triazine-containing melamine wastewaters and the recycling of the purified water to the melamine plant.
According to WO 01/46159 A2, the mother liquor comprising melamine and OATs is acidified up to pH=7 after the melamine crystallization, which forms an OAT suspension which is then subjected to a tangential filtration. This affords an aqueous melamine solution and a OAT dispersion. While the aqueous melamine solution is recycled into the process, the OATs are separated from the dispersion.
A further means of wastewater treatment in melamine plants consists in treating the wastewaters in a thermal wastewater treatment plant (TAA), where the triazine-containing wastewater ingredients are hydrolysed under high pressure and high temperature in the liquid phase to CO2 and NH3. Such a process is described, for example, in IT 01282370. There, the crystallization mother liquors of a melamine plant are heated to 180 to 250° C. in a closed vessel under the autogenous pressure of the system, subsequently cooled down, CO2 is added till a pH-value of 6-8 is reached and the thus formed precipitation is separated off from the treated mother liquid.
In a similar manner, according to IT 01282369, triazine-containing melamine wastewater is treated in a closed vessel at a temperature of >250° C. The NH3 and CO2 formed is subsequently stripped off and the resulting pure liquid is recycled into the plant or discharged.
WO 02/100839 A1 describes a process in which the quenching process is conducted by using NH3 containing water. The majority of the mother liquors of the melamine crystallization are recycled untreated into the melamine plant; NH3 and OAT are recovered from the smaller portion of the mother liquors. The disadvantage of this process is that more fresh water additionally has to be supplied when the untreated, OAT-containing mother liquors are recycled into the melamine process in order to be able to work up the crude melamine melt to the desired quality.
WO 2006/042760 A1 provides a process for treating triazine-containing water of a melamine plant whereby the water is fed to at least one membrane filtration unit (MF), the water is separated in the membrane filtration unit (MF) into a fraction rich in ionic triazines and a fraction rich in nonionic triazines, and then the fraction rich in ionic triazines is discharged and the fraction rich in nonionic triazines is recycled into the melamine plant.
WO 2006/133966 provides a thermal process for cleaning wastewaters of a melamine plant, in which triazine-containing wastewater is subjected to a thermal pretreatment stage to form a gas phase and a liquid phase, the liquid phase of the thermal pretreatment stage is subjected to a thermal hydrolysis stage and finally NH3 is removed from the resulting H2O-, CO2- and NH3-containing liquid phase in a stripping stage. The thus obtained NH3-free liquid phase is either discharged or recycled. In the specific case that the aqueous workup part of the melamine plant proceeds in the presence of NaOH, the H2O-rich bottom phase contains sodium carbonate and can therefore not be recycled into the melamine process.
The quality of the recycled waste water e.g. pH or salt content depends strongly on the applied treatment methods. Thus, it is still often necessary to add fresh water and/or fresh alkaline solution to the wet process in order to maintain the desired quality of the produced melamine.